One step in the fabrication of modern electronic devices is the formation of a conductor pattern over a substrate through photolithographic masking and etching. This step employs a photoresist coating over a metal layer which is selectively exposed to actinic light through a mask defining the desired conductor pattern. The photoresist film is developed so that it, in turn, constitutes a mask having openings defining the regions of the conductor to be etched.
One method of assembling integrated circuit devices is to support a number of them on a ceramic substrate to form an assembly known as a hybrid integrated circuit. It is sometimes advantageous to use an electrophoretic resist as the photoresist for patterning the metal conductor pattern on the substrate of a hybrid integrated circuit. An electrophoretic resist is typically an organic chemical, usually polymethylmethacrylate based, which is deposited on the metal coating by an electroplating process. Such resists offer advantages of fine-line photolithographic definition as well as physical robustness. After masking and etching of the metal layer to form the conductor pattern, the electrophoretic mask must be removed using a photoresist stripping solvent. A commonly used stripping solvent for the electrophoretic resists is methoxy ethanol in two percent KOH, which is composed mostly of a glycolether component. Unfortunately, glycolether is a toxic teratogen, which constitutes both a hazardous liquid waste and a hazardous air pollutant.
Efforts at replacing glycolether-based solvents have proven difficult because cured electrophoretic resist is resistant to many solvents, and because many of the alternative solvents are difficult or disadvantageous to use. Certain alternatives, for example, require application at a high temperature which may cause harmful thermal stresses. Other alternatives damage polymers which may be part of a hybrid integrated circuit substrate. Still other alternatives have relatively low flash points which may make them dangerous to use.
There is therefore a long-felt need for safe and effective alternatives to glycolether-based solvents for stripping electrophoretic photoresists.